1. Field of the Invention
The present invention relates to optical detectors. More specifically, the present invention relates to systems and methods for simultaneously detecting multiple frequency bands.
2. Description of the Related Art
Imaging systems typically use an array of detector elements to generate an image of a target area. Each individual detector element measures the intensity of energy (such as infrared or visible light) incident upon the detector element, and this measurement is then used to form one pixel of the output image.
Conventional infrared imaging arrays typically have a single detector element per pixel and can only detect energy in one frequency band. In certain applications, however, it would be advantageous to be able to simultaneously image multiple frequency bands. For example, an important application of imaging devices is infrared imaging of military targets. The ability to simultaneously image multiple frequency bands could make it easier to identify targets via their infrared signatures.
Prior approaches to detecting multiple frequency bands include filter arrays, dispersive systems, and vertically stacked photodiodes. Filter arrays are commonly used in color digital cameras, in which conventional color filters (such as absorptive glass filters or dichroic thin film filters) are placed over the detector elements to perform spectral discrimination, typically using a 2×2 grid pattern of red, green, green, and blue filters. Since each detector element is filtered to record only one of three colors, two-thirds of the color data is missing from each pixel. (A demosaicing algorithm may be used to interpolate red, green, and blue values for each pixel.) This filter array approach may be applied to infrared systems; however, it is inefficient because much of the power incident on the detector array is thrown away (since each element is filtered to detect only one wavelength).
A dispersive system uses one dimension of a two-dimensional array to look at wavelength. The other dimension of the array can be used to obtain one spatial dimension. The array is then scanned to obtain the second spatial dimension. Thus, the system generates a spectrum for each pixel of the two-dimensional image. Scanning in one dimension, however, results in loss of sensitivity with respect to a staring array.
A two-color photovoltaic pixel uses a vertical stack of two p-n junctions, the one nearer the front (with respect to the direction in which the light is propagating) having a wider band gap, or cut off at a shorter wavelength. This approach can be configured to simultaneously look at two different frequency bands. However, it is difficult to scale to more than two bands.
Hence, a need exists in the art for an improved system or method for simultaneously imaging multiple frequency bands that is more efficient and more sensitive than prior approaches.